1. Technical Field of the Invention
The present invention relates to mobile terminals such as digital land mobile radiotelephones and pocket telephones, and particularly to mobile terminals for code division multiple access (CDMA) or CDMA frequency division duplex (CDMA/FDD).
2. Description of the Prior Art
The diversity is used for overcoming the fading or a fluctuation of a level of received electromagnetic wave due to interference by the waves which propagate through various routes.
In the reception diversity, the strongest component is selected among the components received by a plurality of antennas, or several components are selectively combined.
Further, in the transmission diversity, electromagnetic waves are transmitted by a plurality of antennas of which directivities are different from each other.
Furthermore, in the time division duplex (TDD) wherein a single frequency is used for transmission and reception, the transmission diversity is executed by selecting an antenna by which the mobile station received the strongest wave and by transmitting by using the selected antenna.
However, the transmission diversity in the TDD can not be employed for the frequency division duplex (FDD), because the frequency for an upward link is different from that for a downward link in FDD, and therefore the upward fading is different from the downward fading.
Therefore, in FDD, the closed loop control is required for controlling a level and phase of the wave transmitted by a base station, on the basis of a control command from mobile stations. In this case, information symbols or pseudo noise codes are different antenna by antenna.
Concretely, 0.625 ms time slot is used in the closed loop control, thereby the mobile terminal""s measuring propagation states from each antenna of the base station, and thereby the base station""s determining an antenna for transmission, or transmission ratio and phases for each antenna. For example, antenna 1 may be selected on the basis of a command from a mobile terminal. Antenna 2 may be selected, when the mobile station prefers antenna 2. Thus, the base station changes the antenna after the slot next to the slot which instructs the antenna selection.
The ratio of transmission levels and phases of the transmission waves are also changed by the control command from the mobile station.
Furthermore, when four bits in a slot are available for the control command in a system wherein the control command is transmitted every four slots, one bit may be used for the transmission rate, while the three bits may be used for eight phases. Thus, the control command controls the base station precisely.
The transmission diversity as disclosed in JP 8-195703 A (1996) employs a plurality of orthogonal codes such as WALSH function in order to communicate with all the mobile stations. The mobile stations despread the signals from the base station, and select one of them or combine two or more of them. The transmission power can be reduced due to the transmission diversity.
In the CDMA/TDD accompanied by TDMA as disclosed in JP 11-275035 A (1999), a sub-frame consisting of a plurality of slots is used for CDMA/TDD-TDMA, in order to maintain controllability of transmission power and transmission diversity. Concretely, the mobile terminal is provided with a signal interference ratio (SIR) measurement unit for monitoring a notification channel in the downward channel, while the base station is provided with a transmission power control unit for controlling the upward transmission power on the basis of the measurement result by an SIR measurement unit. Here, for example, an asymmetric communication by CDMA/TDD is executed by using one slot for the upward and three slots for the downward. In this case, all the sub-frames consisting of four slots allocated for all the users are transmitted by TDMA.
However, the base station may possibly confuse the control command from the mobile terminal, when the upward link is not in a good condition. For example, the base station may execute a transmission by using antenna 2, in spite of the command from the mobile terminal which orders the base station to use antenna 1. In this case, communication qualities may become worse.
On the other hand, when the downward link is not in a good condition, the mobile terminal may not recognize correctly the transmission channel, even when the base station works according to the control command. For example, the mobile station may recognize that the base station executes a transmission by antenna 2, in spite of the fact that the base station is using antenna 1. As a result, the mobile terminal can not demodulate the signal from the base station, because information symbol or pseudo random code differs antenna by antenna.
An object of the present invention is to control the transmission diversity by monitoring the upward and downward link.
The mobile terminal of the present invention transmits a control command for controlling a base station with transmission diversity by using two or more antennas, thereby enjoying a good downward link. The mobile terminal of the present invention includes:
a reception unit for receiving downward signal from the base station;
a channel estimation unit for estimating a downward channel on the basis of received downward signal;
a channel prediction unit for predicting a downward channel which is used at the time when the base station is controlled by the control command;
a control command generation unit for generating the control command on the basis of the predicted downward channel;
a mixer unit for mixing the generated control command and information data from the mobile terminal; and
a transmission unit for transmitting the output from the mixer unit.
The mobile terminal of the present invention receives signals from the base station which receives the control command from the mobile terminal and executes on the basis of the control command the transmission diversity by using two or more antennas, thereby enjoying a good downward link. The mobile terminal of the present invention includes:
a reception unit for receiving downward signal from the base station;
a downward link estimation unit for estimating the downward channel on the basis of the received downward signal;
an upward link estimation unit for estimating the upward channel on the basis of the received downward signal;
a base station estimation unit for estimating a state of the base station on the basis of the outputs from the downward estimation unit and the upward estimation unit; and
a demodulator unit for demodulating the received signal on the basis of the output from the base station estimation unit.
The downward link estimation unit in the mobile terminal of the present invention estimates the downward link on the basis of a signal level, or SIR of the received signal.
The upward link estimation unit in the mobile terminal of the present invention estimates the upward link on the basis of a transmission power control command from the base station.
The base station estimation unit in the mobile terminal of the present invention on the basis of the estimated downward link, when the downward link is in a good condition and the upward link is in a bad condition. On the contrary, the base station estimation unit estimates that the base station executes a transmission in accordance with the control command from mobile terminal, when the downward link is in a bad condition and the upward link is in a good condition.
According to the present invention, The transmission state of the base station can be grasped correctly, even when the upward link and/or downward link is in a bad condition. Therefore, the mobile terminal receives good quality signals and enjoys a merit of the transmission diversity.